Serial distribution of airway mechanical properties in dogs: effects of histamine

We measured respiratory input impedance (Zin; 8-2,048 Hz) in five dogs (anesthetized, tracheostomized, vagotomized, and mechanically ventilated) during 80 s of apnea after a bolus intravenous injection of saline or histamine (5.0 mg). In the control case, three antiresonances in Zin were found in four of the dogs, whereas in the remaining dog only two were found. The magnitude and frequency of these antiresonances were significantly altered after bronchoconstriction. To interpret Zin, a model incorporating detailed airway geometry, asymmetrical branching, and nonrigid airway walls was developed. The model fit both the saline and histamine Zin data well and predicted a serial distribution of bronchoconstriction consistent with known effects of histamine; i.e., the diameters of the most peripheral airways were reduced (26% of their control values), whereas tracheal diameters were not significantly affected. The model provided estimates of tracheal diameters that were well correlated (r = 0.92) with direct measurements. Control estimates of soft tissue viscosity (1.63 +/- 0.42 cmH2O.s) and Young's modulus (406 +/- 125 cmH2O) compared closely with values in the literature. These results indicate that bronchoconstriction induced by histamine results in significant changes in Zin over this frequency range and that by using this data analysis approach definitive physiological parameters relative to airway geometry and wall mechanical properties can be obtained from measurements made at the airway opening.